1. Field of the Invention
The present invention relates generally to a circuit for protecting an electronic circuit from environmental noise. More specifically, the invention relates to a protection circuit for an input of a semiconductor device fabricated as an integrated circuit (IC) ship.
2. Description of the Background Art
Electric Science Series, Technical Report of CMOS Application, pages 47 to 48, published on 1980, by Sanpo Shuppan, discloses a protection circuit for an electronic circuit. The proposed circuit comprises a protection resistor, a capacitor and diodes for clamping the voltage. The protection circuit is fabricated as an external circuit of the electronic circuit and interposed between an input signal source and an input of the electronic circuit. The input signal source includes a pull-up resistor and a signal generating switch. A potential at a junction between the pull-up resistor and the signal generating switch, which potential is variable depending upon the position of the signal generating switch, serves as input for the electronic circuit. The protection circuit is generally designed for removing noise, such as surge voltage, electric wave distortion or chattering in switching, superimposed on the signal on a connection from the signal source to the input of the electronic circuit. In the prior proposed protection circuit, the noise component is removed by filtering the signal input to the electronic circuit by a time constant circuit determined by the capacity of a capacitor and the resistance of the protection resistor. Furthermore, the protection circuit is designed for protecting the electronic circuit from overcurrent and/or overvoltage which otherwise causes destruction of the electronic circuit.
In practical application, the protection circuit will be required to be capable of performing various. For example, in the case of the protection circuit applied to an automotive electronic circuit, the following performances are required.
(1) It is required to absorb several hundred volts of surge energy which can be induced in various loads. PA0 (2) It is required to remove wave distortion noise due to environmental electromagnetic waves or high frequency noise pulses due to surging. PA0 (3) As an additional required performance of the protection circuit for an automotive electronic circuit, it is required to have the capacity for maintaining sufficiently high contact current, e.g. normally 1 mA to 2 mA, for breaking an oxidation layer which is provided for protecting wiring from shorting otherwise caused by vibration, temperature or humidity in automotive use.
Such a protection circuit is very important for the durability of the electronic circuit. In view of this, a further high performance protection circuit has been required. Furthermore, for such a protection circuit requiring additional components for fabricating the electronic circuit, reduction of the fabricating cost and size is an extremely important task.
In order to answer the requirement for reduction of size, it has been proposed to form the protection circuit as an integrated circuit (IC). This may be particularly advantageous in employing an IC type protection circuit in the case when a plurality of input signal sources are connected to the electronic circuit. On the other hand, in view of production cost, it is preferred to fabricate the IC chip for the protection circuit as a monolithic IC instead of a hybrid IC, and it is further preferred that it be produced through an ordinary CMOS standard process without requiring a special process.
In practically fabricating the IC protection circuit, it is preferred to incorporate the pull-up resistor in IC form but to externally provide the protection resistor to which several hundreds volts is to be directly applied. For this purpose, there has been proposed a CMOS protection circuit as an IC circuit.
In such a CMOS protection circuit, the protection resistor is provided as an external component and the pull-up resistor is incorporated in the IC chip forming part of the circuit on the IC.
Such a prior proposed protection circuit is difficult to practically fabricate and to practically use. Namely, the resistance of the protection resistor has to be large enough to prevent the diodes in the protection circuit from causing latching-up. Assuming the possible surge voltage is .+-.300, and the current flowing through the diodes is greater than or equal to 100 mA, the required resistance R of the protection resistor can be illustrated by: EQU 300 (V)/R.ltoreq.100 (mA) EQU R.gtoreq.3 (k.omega.) (1)
On the other hand, in order to assure breakage of an oxidation layer, the sum value of resistances of the protection resistor and the pull-up resistor is required to satisfy the following condition:
Assuming the contact point current of the IC is greater than or equal to 1 mA the power source voltage is 5 V, the sum value of the resistances of the protection resistor and the pull-up resistor is illustrated by: EQU 5 (V)/(R+r).gtoreq.1 (mA) EQU R+r.ltoreq.5 (k.omega.) (2)
From foregoing formula (1) and (2), the required resistance r of the pull-up resistor becomes smaller than or equal to 2 k.omega..
As can be appreciated herefrom, in the certain applications, it becomes necessary to set the resistance R of the protection resistor at a substantially greater value than that of the pull-up resistor. This makes it difficult to detect the state of the signal generating switch. Namely, when the signal generating switch is OFF, the input voltage becomes the source voltage Vcc and when the signal generating switch is ON, the input voltage becomes (R.times.Vcc)/(R+r). Therefore, when the resistance R of the protection resistor is greater than the resistance of the pull-up resistor, the voltage difference between the switch OFF state and ON state becomes smaller. This causes difficulty in setting a threshold level for detecting the ON and OFF states of the signal generating switch.
This problem is particularly serious when a buffer circuit is provided. Namely, in a normal case, the logical threshold voltage Vth of a CMOS buffer circuit is set at approximately half of the source voltage Vcc. This means that the input voltage has to be lower than half of the source voltage Vcc when the switch is in the ON state in order to allow detection of the ON state of the switch. For this, when the resistance R of the protection resistor is set to be greater than the resistance r of the pull-up resistor, the input voltage at the ON state of the switch becomes greater than the threshold voltage Vth. Therefore, in such case, detection of the state of the signal generating switch becomes impossible.
In order to solve this problem, the pull-up resistor has to be made as an external component of the IC chip. However, this clearly increases the complexity of the production process and the parts required for fabricating the IC chip. In other approach, it may be possible to solve the above-mentioned problem by applying a higher source voltage so as to increase the contact point current for assuring breakage of the oxidation layer. However, in a normal case, when the regulated power source for the electronic circuit is set at 5 V, another high voltage power source becomes necessary thus increasing the cost and causing chip size bulk.